1. Field of the Invention
The present invention relates to a method of manufacturing a ferroelectric liquid crystal element used in, e.g., graphic display.
2. Related Background Art
A surface stabilization ferroelectric liquid crystal (to be referred to as an SSFLC hereinafter) is realized in such a manner that a ferroelectric liquid crystal (to be referred to as an FLC hereinafter) is clamped in a gap between two substrates, and the substrate gap is sufficiently decreased to cause a helical structure of the FLC to disappear. The SSFLC is often utilized in a chiral smectic C phase. As is known, in the smectic phase, liquid crystal molecules form a layered structure. In the SSFLC, the layers have alignment structures shown in FIGS. 2A to 2D. The layered structure shown in FIG. 2A is normally called a bookshelf type. In this structure, the layers are perpendicular to substrates 21 and 22. The structure of this type can be formed by applying a strong alternating electric field to an SSFLC having one of the alignment structures shown in FIGS. 2B to 2D. The structure shown in FIG. 2B is of inclined bookshelf type, and appears when substrates subjected to an alignment treatment with a large pre-tilt angle are adhered to each other in an anti-parallel state. The structures shown in FIGS. 2C and 2D are of a so-called chevron type, and appear when substrates subjected to a rubbing treatment are adhered to each other so that rubbing directions A of the upper and lower substrates are almost parallel to each other. The type shown in FIG. 2C is stable at a high temperature, and the type shown in FIG. 2D appears through zig-zag defects in a process wherein the temperature of the type shown in FIG. 2C is decreased.
The relationship between the layered structures shown in FIGS. 2C and 2D and a temperature is known to those who are skilled in the art. Alignment states of the type shown in FIG. 2C include four surface stabilization states. Two out of these four states are so-called spray alignment states wherein directors are twisted between the upper and lower substrates. The remaining two states are so-called uniform alignment states wherein directors are almost uniform between the upper and lower substrates. It is presumed that these alignment states correspond to alignment states shown in FIG. 3 under an assumption that directors are present on a cone defined by the helical structure of the smectic C phase. In FIG. 3, a to d represent arrangements of C directors when the cone is viewed from its bottom surface. More specifically, a and b indicate the spray alignment states, and c and d indicate uniform alignment states. An arrow 31 indicates the direction of spontaneous polarization. Of these surface stabilization states, the uniform alignment states c and d are used in a display element, thereby realizing a high-contrast display.
However, when the uniform alignment state is used as a display element, a so-called "printing" phenomenon often occurs. In this phenomenon, when the display element set in one display state is kept driven for a long period of time, the display content is fixed, and slightly remains even when the display content is rewritten. In order to prevent this phenomenon, the present inventors found that alignment treatment substrates having a large pre-tilt angle could be used. However, when the pre-tilt angle is increased, another problem to be described below is posed.
The FLC is normally used in the form of a simple matrix display element. In a popular drive method in this case, as disclosed in Japanese Laid-Open Patent Application No. 60-33535, a voltage equal to or higher than a threshold value is applied to pixels on a selected scanning line to determine an ON or OFF state, and an alternating voltage signal is applied to pixels on a non-selected scanning line. However, when an identical display state is maintained for a long period of time in the uniform alignment state in this drive method, image quality is degraded, and the display element cannot withstand long-time use.